We have detected an unusually large structure (which we term "Sm cluster") in the nucleus of HeLa cells stained with a monoclonal anti-Sm antibody and examined by electron microscopy. One of our goals is to study this structure further. The possibility of detecting more than one Sm cluster per nucleus will be examined by serial sectioning (electron microscopy) or light microscopy of whole cells. The frequency and appearance of the Sm clusters will be examined in several situations: heat-shocked versus normal temperature Drosophila and HeLa cells (to block hnRNP assembly); digestion of cells with pancreatic RNAase; growing versus resting cells; cell exposure to inhibitors of protein synthesis; dimethyl sulfoxide-induced versus noninduced Friend erythroleukemia cells; cells from species other than human (fruit fly, frog, chicken); additional staining probes such as other polyclonal anti-Sm and anti-RNP antibodies and antibodies to trimethylguanosine. The isolation of Sm clusters is another objective. Whole nuclear and whole cytoplasmic fractions from human somatic cells were analyzed in this laboratory by protein gel blot analysis using a monoclonal anti-Sm antibody and polyclonal anti-(U1)RNP antibodies as probes. Unexpectedly, some cytoplasmic proteins reacted with these antibodies. In addition, these antibodies recognized some nuclear proteins, which are different both from the nuclear proteins reported in the literature and from the cytoplasmic proteins just mentioned. Further studies on these polypeptides are planned. (H)